The present invention relates to gas analysis by means of microwaves and more particularly, the invention relates to an electric circuit for operating a cavity resonator in a microwave gas analyzer.
A gas analyzer of the type referred to above is disclosed, for example, in U.S. Pat. No. 2,792,548. The device includes a cavity resonator and a microwave generator applying microwave energy to the resonator, and the cavity of the latter receives also the gas to be analyzed. The resonator is tuned to the specific absorption frequency of the particular component to be detected, and the attenuation of the tuned frequency waves in the resonator is used as representation of the concentration of that gas component.
If the resonator of such an analyzer is not driven at exactly its resonance frequency, the power through-put in the resonator will, change and any measurment is no longer adequately representative or even outright faulty. Such changes in frequency can readily occur, e.g. on account of temperture drift, mechanical vibrations, etc., unless one controls the frequency output of the microwave generator to track the changing resonance frequency in the cavity.
High performance analyzers for high sensitivity of measurement are correspondingly very sensitive to any change or difference between generator and resonator frequencies. Such differences are already noticeable even if considersably smaller than the band width of the abosrption line of interest. Thus, the frequency deviation must remain well below that band width.
The known circuits for frequency control of a microwave generator and cavity resonator are usually provided with a particular microwave circuit which extracts signals from the resonator and the microwave generator and furnishes a voltage (error signal) repesenting the difference in frequencies. This voltage is amplified and controls the frequency determining element in the generator in a typical closed loop, feedback configuration (see e.g. "Frequency Stabilisation of Microwave Oscillators", R. V. Pound Proc. I.R.E., Vol. 35, pages 1405 to 1415, December 1947). It is inherent that such a control system is rather expensive on account of the need for detecting actual frequencies as they occur or are effective and which are in the microwave range.